This invention relates to AC variable speed drives and to methods of controlling variable speed drives and, more particularly, to a drive system which utilizes synchronous or doubly fed electric motors. The drive system is fed with polyphase AC (alternating current) electric power from a grid and is capable of driving a varying mechanical load while operating substantially at a desired power factor as seen from the power grid, such as unity power factor.
A doubly fed motor is an electrical machine having a rotor with rotor windings and a stator with stator windings. The doubly fed motor receives electrical power on both the stator and rotor windings. The received power may be polyphase AC power on both the stator and rotor windings. In the synchronous motor, polyphase AC power may be received by one set of either the stator or rotor windings, and direct current (DC) power to the other set of windings. The doubly fed motor is capable of variable speed operation, whereas the synchronous motor drives a load at synchronous speed.
Unity power factor is achieved when the waveforms of the polyphase AC voltage and current received from the power grid are in phase, that is, neither waveform is leading or lagging the other waveform. Operation at a poor power factor, either leading or lagging, draws more current from the line than strictly needed by the mechanical load driven by the motor. Operation at a poor power factor also creates unnecessary losses in the power lines supplying the motor as well as in the motor itself. Moreover, the motor has to be designed with a rating higher than required by the mechanical load at the full-load condition. Thus, the initial cost of such a motor, as well as the associated operating costs, are higher if the variable speed drive operates at a non-unity power factor, and unity power factor is a preferred embodiment in most situations. However, some applications may require the motor to be controlled at a desired power factor other than unity.
Previous variable speed drives have used a regulator which requires an error signal of a speed sensor output and a certain desired reference
with variable speed drives, special signal. With other types of motors used arrangements are provided for frequency control of the armature voltage to attain a region of constant allowable torque. Often an additional controllable exciter or an additional power electronic converter is required to compensate for the detrimental effect of the armature reaction, as is required with other synchronous motor drives.
Several critical problems exist in prior variable speed drives using doubly fed motors. One problem, common to all types of AC variable speed drives, is the power electronic converter required to satisfy the adjustable frequency requirements of the motor currents. Most converters generate an excessive content of low-order harmonics in the voltage and current waveforms. Such a converter also produces a poor power factor as seen from the polyphase AC power grid to which the input side of the converter is connected.
However, recent progress in power electronic conversion technology has provided converters which constrain the detrimental low-order harmonics and the poor power factor. For example, the Schwarz converter as described in U.S. Pat. No. 4,096,557 is an effective means for establishing a near unity power factor as seen from the AC power grid.
A known configuration for interfacing the doubly fed motor drive with the AC power grid is to place any type of converter between the power grid and the motor. Both the stator and rotor windings of the motor are connected in opposite phase-polarity to the output terminals of the converter. Thus, the converter must be designed to provide the active, as well as reactive, power requirements of both stator and rotor windings for the entire speed range of the drive. This type of converter design suffers from increased initial and operating costs. Also, this known system suffers from inherent instability problems.
A rotor position feedback has been proposed to solve the inherent instability problem. Rotor position feedback leads to torque-speed characteristics similar to series DC motors. But this approach disadvantageously requires controlling the voltage with frequency to maintain a level of air gap flux adequate to produce maximum torque as required by the load.
The rotor position feedback approach also causes the loss of another attractive potential of the doubly fed motor, which is the speed adjustment flexibility. With rotor position feedback, the control system is constantly correcting for the speed error deviation from a desired reference speed. The time delays associated with this continual correction detract from the responsiveness of the drive system.
Therefore, a need exists for a new drive system, controller, and method for controlling motors at a desired power factor under varying mechanical loads.